Composite precursor for aluminum-containing lithium transition metal oxide and process for preparation of the same

ABSTRACT

The present invention provides a powdery composite precursor, which comprises a core of a lithium transition metal oxide, and an aluminum hydroxide-based precipitate layer coated on the surface of the core, and a process to prepare the composite precursor. The preparation process comprises the formation of a water based slurry by dispersing lithium transition metal oxide powder in water, and a precipitation reaction of an aluminum salt solution with a base solution where the lithium transition metal particles act as seed particles, whereby a mechanically stable precipitate layer of homogeneous thickness can be achieved. The composite precursor can be converted into aluminum-containing, e.g., aluminum-doped, lithium transition metal oxide suitable for a cathode active material of lithium rechargeable battery by heat treatment.

FIELD OF THE INVENTION

The present invention relates to a composite precursor foraluminum-containing lithium transition metal oxide and a process forpreparation thereof. More specifically, the present invention relates toa composite precursor comprising a core of lithium transition metaloxide and an aluminum hydroxide-based precipitate layer coated on thesurface of the core, and a preparation process of the compositeprecursor by precipitation reaction of an aluminum salt solution and abase solution to coat the lithium transition metal oxide, as a seedparticle, in a water-based slurry or paste.

BACKGROUND OF THE INVENTION

In commercial rechargeable lithium batteries, lithiated transition metaloxides are employed as a cathode active material and these transitionmetal oxides include, for example, materials of the layered crystalstructure such as LiCoO₂, Li(Mn_(1/2)Ni_(1/2))_(1-x)Co_(x)O₂, orLiNi_(1-x)Co_(x)O₂, and materials of the spinel crystal structure suchas lithium manganese oxide spinel or lithium manganese-nickel oxidespinel. Depending on the application, certain properties of thesematerials are of importance and such properties can be modified byprocessing, doping, surface treatment, control of impurities, etc.

Some of these materials, particularly where only one type of transitionmetal is present, can be easily prepared by solid state reaction usingsimple transition metal precursors. However, more “complex” materials,particularly where two or more types of transition metals are present,are difficult or impossible to prepare by simple solid state reaction,i.e. by mixing separate transition metal precursors. Instead, complexlithium transition metal oxides are generally prepared by reacting mixedprecursors, e.g., mixed hydroxides or mechanically alloyed transitionmetal oxides, with a source of lithium.

Mixed hydroxides are typically prepared by precipitation reaction.Precipitation of mixed hydroxides (for example, the precipitation of aflow of NaOH with a flow of M-SO₄ under controlled pH) or mixedcarbonates (for example, the precipitation of a flow of Na₂CO₃ with aflow of M-SO₄) allows precursors of suitable morphology to be achieved.A problem is the level of impurities; especially, the removal of sulfuris difficult and expensive. The same problems apply to the removal ofsodium in the case of mixed carbonates.

Meanwhile, as one method for modification of lithium transition metaloxides, doping has been widely investigated. The doping typically doesnot exceed 5% by atoms of dopant per transition metal. Typical dopantsare either inserted isostructurally into the existing crystal structure(e.g., Al-doped LiCoO₂) or they form a secondary phase, oftenagglomerating at grain boundaries (e.g., Zr-doped LiCoO₂).

In the doping approach, aluminum is a general dopant. The benefit ofaluminum-modified lithium transition metal oxides has been widelyinvestigated. For example, it is known that adding Al to the crystalstructure of LiNi_(1-x)Co_(x)O₂ improves safety and cycling stability.For example, Al-doped LiMn_(2-x)Li_(x)O₄ spinel cycles more stably andalso shows less dissolution of Mn, and Al-coated LiCoO₂ cycles morestably at high voltage.

A problem associated with aluminum-modified lithium transition metaloxides, i.e., Al-containing lithium transition metal oxides, is thepreparation process thereof. In the case of complex lithium transitionmetal oxides, mixed precursors would have to contain aluminum; however,it is more difficult to prepare Al-containing precursors such asAl-doped mixed transition metal hydroxide. Alternatively, lithiumtransition metal oxides could be prepared by mixing raw materials with asource of aluminum such as Al₂O₃ or Al(OH)₃. In this regard, it shouldbe noted that Al₂O₃ has low reactivity and Al(OH)₃ is easily transformedto Al₂O₃ at low temperature. Therefore, the obtained cathode isnonhomogeneously doped so that the benefit of aluminum doping is notfully utilized.

In a process for preparation of Al-doped materials, if a layercontaining a reactive aluminum phase were to fully cover the surface ofa subject particle such as lithium transition metal oxide, this would beadvantageous. If this were possible, the diffusion pathway would beshort and the contact area would be large so that an Al-doped materialcould be achieved at relatively low reaction temperature. As will beillustrated later, the present invention discloses such compositeprecursor fully coated with a reactive aluminum phase and a process forpreparation of the composite precursor.

Meanwhile, besides the Al doping approach, an Al coating approach isalso known as a means to improve properties. In a conventional Alcoating process, lithium transition metal oxide particles are dippedinto an aluminum-containing solution or gel, followed by drying and mildheat treatment. As a result, the surface of lithium transition metaloxide is coated by an aluminum oxide-based phase. This phase separatesthe electrolyte from the more reactive bulk and promises improvedproperties. However, the conventional Al coating process has demerits asexplained below.

In the prior art Al coating process, lithium transitional metal oxidesare dipped into AlPO₄ or tri-butyl aluminum dissolved in ethanol.Problems are the cost of raw materials and the use of organic solventsthat may cause the generation of gas during reaction or dryingprocedures. A further problem is that only a small amount of Al can becoated on the lithium transition metal oxide. Low solubility of AlPO₄ ortri-butyl aluminum limits the amount of aluminum present in a layerformed by dip-coating. Where an organic solvent is used in large amountsto compensate for the low solubility, aluminum-containing particlesform, but fail to cover the lithium transition metal oxide. Generally,the contact between the aluminum compound and lithium transition metaloxide after a drying procedure is maintained mainly by physical adhesionand to a lesser extent by chemical bonds. Accordingly, although athicker coating layer is made, it tends to disintegrate during drying.

As an alternative approach, a particle coating process is also known inthe art. In this process, coating is achieved by dipping lithiumtransition metal oxides into a slurry of fine particles. Alternatively,it is also possible to apply a dry coating approach. In this dry coatingprocess, fine powders, typically Al₂O₃ particles of sub-micrometer sizeare mixed with lithium transition metal oxides. However, the particlecoating process has some disadvantages, as follows: (i) it is difficultto achieve a full coverage by fine particles; (ii) it is difficult toprevent agglomeration of fine powders, and the resulting agglomeratesfail to efficiently cover the surface of lithium transition metal oxide;and (iii) the adhesion between fine particles and lithium transitionmetal oxide is poor so that the coating layers tend to peel off duringsubsequent processes.

Therefore, improved precursors for cathode active material and a methodto prepare such precursors are needed. The improved precursors could becharacterized as lithium transition metal oxide with a uniformly thicklayer fully covering the particle, the layer having good mechanicalcontact, containing aluminum and being practically free of impurities.

SUMMARY OF THE INVENTION

The objects of the present invention are to completely solve theproblems described above.

Specifically, an object of the present invention is to provide acomposite precursor for aluminum-containing lithium transition metaloxide as a cathode active material. The composite precursor has analuminum hydroxide-based precipitate layer of active aluminum phase onthe surface of a lithium transition metal oxide core and can beconverted into an Al-doped lithium transition metal oxide by heattreatment.

Another object of the present invention is to provide a process ofpreparing the composite precursor by precipitation reaction of analuminum salt solution and base solution. This precipitation process hasmany advantages; for example, impurities such as sulfur, sodium orchloride present in raw materials can be tolerated, and organic solventsare not used, and a fully coated, stable product can be obtained.

A further object of the present invention is to provide analuminum-containing lithium transition metal oxide able to be producedfrom the composite precursor. The aluminum-containing lithium transitionmetal oxide can be applied to a cathode active material for rechargeablelithium batteries.

In order to accomplish these objects, there is provided in the presentinvention a composite precursor for aluminum-containing lithiumtransition metal oxide comprising (a) a lithium transition metal oxidecore, and (b) an aluminum hydroxide-based precipitate layer of activealuminum phase coated on the surface of the lithium transition metaloxide core, with the aluminum hydroxide-based precipitate containing adivalent anion. This composite precursor is a novel material not knownin the art to which the present invention pertains.

In an embodiment of the present invention, the aluminum hydroxide-basedprecipitate layer is lithium-aluminum-sulfate-hydroxide-hydrate. In thiscase, the divalent anion is sulfate. The sulfate present in theprecipitate layer can be easily replaced by carbonate to allow theprecipitate layer to be practically free of sulfur. Therefore, thealuminum hydroxide-based precipitate layer is preferably oflithium-aluminum-carbonate-hydroxide-hydrate. In this case, the divalentanion is carbonate.

The present invention also provides a process for the preparation of thecomposite precursor, comprising a step of carrying out the precipitationreaction of an aluminum salt solution and base solution with a lithiumtransition metal oxide as a seed particle dispersed in a water-basedslurry or paste to form an aluminum hydroxide-based precipitate layer onthe surface of lithium transition metal oxide particle.

In a preferable embodiment of the present invention, after theprecipitation reaction, an ion exchange reaction is further performed toreplace the sulfate present in the precipitate layer with carbonate byadding a carbonate solution into the reaction system. Severalexperiments described in the present disclosure surprisingly show thatthe ion exchange reaction allows the precipitate layer to be practicallyfree of sulfate.

Also, the present invention provides an aluminum-containing lithiumtransition metal oxide produced by heat treatment of the compositeprecursor as defined above. This aluminum-containing lithium transitionmetal oxide is suitable for a cathode active material of lithiumrechargeable battery, showing more excellent properties than the priorart lithium transition metal oxide and Al-coated or Al-doped lithiumtransition metal oxides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the combination of two X-ray diffraction patterns in which thetop shows the precipitate after gentle drying and the bottom shows thelithium aluminum carbonate hydroxide hydrate obtained after an ionexchange reaction followed by gentle drying.

FIGS. 2A-2C are FESEM micrographs of LiCoO₂ particle coated withlithium-aluminum-carbonate-hydroxide-hydrate after drying at 180° C.,prepared in Example 3.

FIG. 3 is a combination of two graphs for electrochemical cycling (C/5,60° C.), in which the top shows untreated LiCoO₂ and the bottom showsaluminum modified LiCoO₂, carried out in Example 9.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is illustrated below in more detail.

In the composite precursor according to the present invention, thethickness of the precipitate layer is not particularly limited becauseit can be varied depending upon the aluminum content required for anintended aluminum-containing lithium transition metal oxide. In anembodiment, the total amount of aluminum in the precipitate layer is 0.5to 5% by atoms with respect to the total amount of transition metal inthe complex precursor.

As has been mentioned already, the aluminum hydroxide-based precipitatelayer is present as an active aluminum phase which is athermodynamically stabilized phase resistant to dissolving even at highpH, and which can also be easily converted into an Al-doped layer oflithium transition metal oxide by appropriate heat treatment.

The lithium transition metal oxide as the core of composite precursorhas a layered or spinel crystal structure, and includes, for example,but is not limited to LiCoO₂, cobalt-richLi(Mn_(1/2)Ni_(1/2))_(1-x)Co_(x)O₂, lithium manganese spinel such asLiMn_(2-x)Li_(x)O₄ and doped lithium manganese spinel, lithiummanganese-nickel spinel such as Li(Mn_(1.6)Ni_(0.4))O₄, lithiummanganese nickel oxide such as LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂, or lithiumnickel oxide-based materials such as LiNi_(0.8)Co_(0.2)O₂ andLiNi_(0.8)Co_(0.15)A_(0.05)O₂ (A=Mn, Al, MgTi, etc.).

In the preparation process of the composite precursor according to thepresent invention, the lithium transition metal oxide as a raw materialcan be tolerated to contain impurities such as sulfur, sodium, chloride,etc. which are highly undesirable in commercial lithium rechargeablebatteries. Surprisingly, these impurities are removed from the lithiumtransition metal oxide during the precipitation reaction of the presentinvention, which can be seen in Example 10 of the present disclosure.Accordingly, for preparation of the lithium transition metal oxide as araw material, cheaper chemicals can be employed and additionalprocedures for removal of impurities, such as a washing step in theprior art, are not required.

Also, in the process of the present invention, no organic solvents whichmay cause the generation of gas during processing are used, whereaswater which can be easily removed by appropriate separation such asfiltering is used.

In addition, the process of the present invention allows a homogenous,thicker aluminum-containing coating to be achieved, compared to theprior art process. Furthermore, the achieved coating layer has excellentmechanical stability.

In the precipitation reaction process, a water-based slurry is firstprepared containing lithium transition metal oxides particles. The solidfraction of slurry preferably exceeds 30˜50% (w/w).

The aluminum salt as one of the reactants for the precipitation reactionincludes, for example, but is not limited to aluminum sulfate, aluminumpotassium sulfate, aluminum sodium sulfate, etc. Among them, aluminumsulfate is particularly preferred because of its high content of Al,high solubility and large scale availability. In some cases, thealuminum salt can be used in combination with other salts, for example,transition metal sulfate such as cobalt, manganese or nickel sulfate,etc.

The base salt as the other reactant for precipitation reaction includes,for example, but is not limited to lithium hydroxide, sodium hydroxide,potassium hydroxide, ammonium hydroxide, sodium carbonate and the like,or mixtures of two or more thereof. Among them, lithium hydroxide isparticularly preferred because the precipitation reaction is possibleeven at high pH without dissolution of aluminum.

The reaction ratio of aluminum salt and base salt is preferably chosenso that the pH is in the range of 6˜12. According to experiments carriedout by the inventors of the present invention, it was ascertained thatwhen the equivalent of base salt is the same as or more than theequivalent of aluminum salt, no aluminum remains in the liquid part ofthe slurry after the precipitation reaction.

The reaction ratio of these salts and lithium transition metal oxideparticle can be determined depending upon the intended thickness ofprecipitate layer. In other words, where a thin layer is intended, a lowreaction ratio is required. On the other hand, where a thick layer isintended, a higher reaction ratio is required.

As mentioned above, where an aluminum-doped lithium transition metaloxide is required as a cathode active material for lithium rechargeablebattery, it might be preferred to prepare an aluminum-free lithiumtransition metal oxide particle, either in powder or slurry form, andthen fully cover the particle by an aluminum-containing precipitatelayer according to the present invention. The precipitate layer can beconverted into an aluminum-containing, doped layer by heat treatment.

As the lithium transition metal oxide, as mentioned above, used can beLiCoO₂, cobalt-rich Li(Mn_(1/2)Ni_(1/2))_(1-x)Co_(x)O₂, lithiummanganese spinel, lithium manganese-nickel spinel, lithium manganesenickel oxide, or lithium nickel oxide-based materials, etc. In anembodiment, instead of lithium transition metal oxides, raw materialsthereof such as mixed hydroxides, or slurries containing the mixedhydroxides may be used. These mixed hydroxides are, for example, M(OH)₂,carbonates such as MCO₃, or oxohydroxides such as MOOH, wherein Mbasically consists of Mn, Ni, Co, etc.

For example, LiNiO₂-based materials such asLiNi_(0.8)Co_(0.15)Mn_(0.05)O₂, and Ni—Mn oxide materials such asLi(Mn_(1/2)Ni_(1/2))_(1-x)Co_(x)O₂ are typically prepared fromprecipitated hydroxides. In these cases, these hydroxides are β-Ni(OH)₂type in which Ni is divalent and they do not contain a significantamount of anions such as SO₄ ⁻² or crystalline water. Meanwhile,trivalent Al does not fit to the crystal structure of an intendedmaterial. If Al and Ni are co-precipitated, the resulting hydroxidescontain counter anions such as sulfate and crystalline water.Accordingly, it is obvious that these hydroxides are less desirable fora large scale process.

The lithium transition metal oxide in the preparation process of thepresent invention is preferably a powder of monolithic particles.Monolithic is defined as particle having a small inner porosity and thetypical example thereof is a potato-shaped or coarse particle. After theprecipitation reaction, a complete, homogenous layer of precipitatecovers the monolithic particle. If particles are porous, lessprecipitate covers the interior of the particle. Alternatively, thelithium transition metal can be a powder consisting of secondaryparticles, being dense agglomerates of smaller primary crystallites;however, it is recommended that the inner porosity is not too large.

The precipitation reaction is conducted in a precipitation vessel as areactor containing the lithium transition metal oxide slurry. Forexample, at least one flow containing an aluminum salt solution and atleast one flow containing a base solution are fed to the reactor.Preferably, both flows are injected continuously and the pH is adjustedto the range of 6˜12. In a preferable embodiment, the aluminum salt isaluminum sulfate and the base is lithium hydroxide. It should be notedthat where LiOH is used as a base, precipitation is possible even athigh pH. It is assumed that the existence of a thermodynamicallystabilized phase—lithium aluminum sulfate hydroxide hydrate—reduces thetendency of aluminum to dissolve at high pH.

During the precipitation reaction, a layer of aluminum hydroxide-basedmaterial precipitates onto the surface of lithium transition metal oxideparticle. As a result, an aluminum hydroxide-based layer of homogeneousthickness, completely covering the surface of lithium transition metaloxide particle, is achieved. The aluminum hydroxide-based layer is notof simple Al(OH)₃ but lithium-aluminum-sulfate-hydroxide-hydrate.

As mentioned above, an ion exchange reaction may be further performedusing a carbonate solution after the precipitation reaction. Forexample, a clear Li₂SO₄ solution, as a solution produced after theprecipitation reaction, is removed and then replaced by a carbonatesolution.

The carbonate for ion exchange reaction includes, for example, but isnot limited to lithium carbonate, sodium carbonate, ammonium carbonate,potassium carbonate or the mixture of two or more thereof. Among them,the lithium carbonate is most preferable. The lithium carbonate can beused in an aqueous form of low concentration, for example 0.1M Li₂CO₃.When the lithium carbonate is used, a homogeneous layer oflithium-aluminum-carbonate-hydroxide-hydrate is obtained. However, itwas confirmed that using hydroxides or other salts instead of thecarbonate were much less effective to provide the sulfate-freeprecipitate layer in the present invention, which can be seen in Example5 of the present disclosure.

After the precipitation reaction or the further ion exchange reaction,the coated particle, more specifically, particle coated with aluminumhydroxide-based layer, is separated and/or washed, followed by drying.

In an embodiment, before or after drying, any chemical can be furtheradded. The chemical includes, for example, but is not limited to LiPO₃,which is added preferably in an aqueous form. In this case, the porosityof aluminum hydroxide-based layer can act like a sponge, thus supportinga homogeneous distribution of the added liquid within the particle.

In anther embodiment, after drying, the resulting powder may be furthermixed with any other chemicals. The chemical includes, for example, butis not limited to Li₂CO₃, Li₃AlF₆, etc., which is mixed preferably in asolid powder form.

An aluminum-containing lithium transition metal oxide according to thepresent invention can be made by performing a heat treatment of thecomposite precursor as prepared above at a temperature sufficient tosinter the lithium transition metal oxide. The temperature of heattreatment is typically in the range of 500˜1050° C. At such sufficientlyhigh temperature, the Al enrichment of the surface vanishes due to fastbulk diffusion and Al-doped lithium transition metal oxide is achieved.However, if the heat treatment temperature is too low, the surfaceremains Al rich and the bulk is less doped or undoped. A desirable heattreatment temperature to achieve a dense surface without excessive bulkdiffusion of aluminum is 700˜950° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in more detail byExamples, but the scope of the present invention is not limited thereto.

Example 1

In the present example, the precipitate of aluminum salt and base saltwas investigated. A flow of 1.33M Al₂(SO₄)₃ solution and a flow of 4MLiOH solution were continuously fed at a constant rate to a reactorcontaining 200 ml of H₂O during agitation. The temperature was 60° C.and pH was 9.0 when measured at 40° C. The total amount of aluminum was0.05 mol. The precipitate was washed by decanting, filtered and dried at60° C. in dynamic vacuum. The chemical analysis on the decanted andfiltered solution surprisingly showed that no aluminum remained in thesolution.

The precipitate was investigated by ICP for the contents of Al and Liand by X-ray to investigate the crystal structure. The content of sulfurwas estimated by EDS. The result showed that the precipitate isbasically of aluminum hydroxide, additionally containing lithium andsulfur. The composition of Al, Li and S is approximatelyAl_(0.7)Li_(0.3)(SO₄)_(0.2). The X-ray diffraction pattern is disclosedin FIG. 1. The X-ray pattern shows a single-phase material.Particularly, the crystal structure is different from Al(OH)₃. Sulfurand lithium are part of the crystal structure and they do not exist as asecond phase (Li₂SO₄), which explains why further washing was noteffective to remove lithium and sulfur. As a result, it is confirmedthat, after the precipitation reaction, alithium-aluminum-sulfate-hydroxide-hydrate had been formed.

Then, an ion exchange reaction was performed by aging a slurrycontaining fresh precipitated lithium-aluminum-sulfate-hydroxide-hydratein a 0.1M Li₂CO₃ solution in which the molar ratio of CO₃:SO₄ wasadjusted to approximately 5:1. The X-ray pattern obtained after the ionexchange reaction resembles that oflithium-aluminum-carbonate-hydroxide-hydrate (Li₂Al₄(CO₃)(OH)₁₂*3H₂O).

Example 2

A commercial lithium manganese spinel (Mitsui) was used as seedparticles. A slurry was prepared by adding 300 ml of H₂O to 250 g oflithium manganese spinel. A flow of 1.33M Al₂(SO₄)₃ solution and a flowof 4M LiOH solution were continuously fed at a constant rate to areactor containing the slurry. The temperature was 80° C. and pH was 9.9when measured at 50° C. 2.5 atom % Al was precipitated per 1 Mn inspinel.

The surface structure of the spinel obtained after washing and dryingwas investigated in the same manner as in Example 1 to confirm that thesurface of the spinel is covered by an aluminum hydroxide-basedprecipitate. EDS investigation showed that the precipitate containedapproximately 25% by atoms of sulfur relative to precipitated Al.

Example 3

LiCoO₂ having the monolithic particle morphology was used as seedparticles, and a slurry containing 2 kg of LiCoO₂ in 1 L of water wasprepared. About 200 ml of the slurry was fed to a reactor. A flow of1.33M Al₂(SO₄)₃ solution and a flow of 4M LiOH solution werecontinuously fed to the reactor at a constant rate. 2% by atoms of Alwere precipitated relative to Co.

In addition, several preparations were carried out under variousreaction conditions, i.e., temperatures varying from 20 to 90° C., pHvarying from about 8 to 11, varying flow rates, etc., so that theprecipitation reaction was finished after 4, 10 or 20 minutes. In somepreparations, 1 mol of Li₂SO₄ per liter water was initially added to theslurry.

The experiments showed that in all cases, a homogeneous layer ofAl-hydroxide-based precipitate, more specificallylithium-aluminum-sulfate-hydroxide-hydrate, fully covering the LiCoO₂surface was formed. The FESEM micrographs showing such coated particlesare disclosed in FIGS. 2A˜2C. The experiments also showed that themorphology of the precipitate layer can be easily controlled,particularly, in view of a higher or lower density, and differentcrystallite size, etc.

Example 4

The experiment was conducted in the same manner as in Example 2 exceptthat a mixed solution of aluminum sulfate and cobalt sulfate was usedinstead of an aluminum sulfate solution. The experimental result showedthat the surface of the lithium manganese spinel was fully covered by acobalt-aluminum mixed hydroxide layer with homogenious thickness, whichcontained additionally sulfate and lithium.

By EDS analysis, the molar composition ratio of Al:Co:S was measured tobe approximately 0.8:1:0.25. The lithium content was not quantified.

Example 5

The experiment was conducted in the same manner as in Example 3 exceptthat a slurry was prepared from 200 g of commercial LiCoO₂ and 150 ml ofwater, and the temperature was adjusted to 60° C., and pH was adjustedto 8.9 when measured at 40° C. The obtained sample had a precipitatelayer formed on the surface of LiCoO₂. The layer contained 3% by atomsof Al per 1 Co atom.

After the precipitation reaction, the sample was washed by decanting anddivided into beakers, then low concentration solutions of suitablesalts, i.e., Li₂CO₃, Na₂CO₃ (approximately 0.03M), LiOH, NaOH, LiF(approximately 0.06M) and Li₃AlF₆ (0.01M), respectively, were addedthereto. After 24 h, the resulting samples were washed and dried.

The sulfur content and the degree of aluminum dissolution were measuredby EDS analysis. The analysis result surprisingly showed that Li₂CO₃ andNa₂CO₃ effectively remove sulfur without causing Al dissolution, whereasLiOH and NaOH were less effective to remove sulfur and a portion ofaluminum was dissolved, and furthermore LiF and Li₃AlF₆ failed to ionexchange sulfur.

Example 6

A slurry was prepared from 4 kg of commercial LiCoO₂ and 2 L of water,and sulfuric acid was added thereto to neutralize the slurry. At 60° C.,a flow of 1.33M Al₂(SO₄)₃ solution and a flow of 4M LiOH solution werecontinuously fed to the 5 L reactor containing the slurry withagitation. The precipitation reaction was continued for 25 minutes withpH being adjusted to 9.6. As a result, a precipitate layer of 2% byatoms of Al relative to Co was formed.

After the precipitation reaction, the Al-coated LiCoO₂ was washed byrepeated decanting. The reactor was refilled with water and 37 g ofLi₂CO₃ was added and dissolved with gentle stirring. After 10 hours, theresulting LiCoO₂ was washed by decanting, followed by filtering anddrying.

EDS analysis showed that LiCoO₂ coated withlithium-aluminum-carbonate-hydroxide-hydrate and substantially free ofsulfur is obtained. More specifically, approximately 2% by atoms ofsulfur were present for every Al atom present in the precipitate layer.

Example 7

The experiment was conducted in the same manner as in Example 6 exceptthat the coated LiCoO₂ after washing by decanting and filtering was notdried. More specifically, instead of the filtercake being placed into abaker, a small amount of water was added until a slurry of highviscosity was achieved. 4.5M LiPO₃ solution was dropped in whileagitating the slurry, which was then dried. In summary, 2% by atoms of Pwas added relative to cobalt atom.

As a result, LiCoO₂ coated withlithium-aluminum-carbonate-hydroxide-hydrate was additionally coatedwith 2% glassy LiPO₃.

Example 8

The Al—Li—OH—CO₃ coated LiCoO₂ of Example 6 was mixed with Li₂CO₃ at anamount of 0.4 g Li₂CO₃ per 100 g LiCoO₂. The mixture was heated toeither 750, 800, 850, 900 or 950° C.

FESEM analysis showed that the degree of sintering and diffusion of Alinto the particle can be easily controlled by correct choice ofsintering temperature. Specifically, at 950° C., a merely Al-dopedLiCoO₂ with a smooth surface was achieved, whereas at the lowesttemperature, a structured, electrically insulating layer of aluminumoxide covered the surface. At intermediate temperatures, a dense, poorlystructured layer based on strongly Al doped LiCoO₂ covered the entiresurface.

As additional experiments, 100 g samples of the Al—Li—OH—CO₃ coatedLiCoO₂ of Example 6 were mixed with 1.2 g of a ballmilled mixture ofLi₂CO₃ and Li₃AlF₆ (1:2 by weight), followed by heating to 850, 900 or930° C. Herein, the fluorine salt acts as a sintering additive (loweringthe sintering temperature required to achieve a certain degree ofdensification) as well as a component supplying an additional protectivelayer of LiF after the sintering.

Example 9

The product of Example 3 was heated to 700° C. Using the heat-treatedproduct as a cathode active material, coin cells were fabricated incombination with Li metal anode to perform the electrochemical test at60° C. Charge and discharge rate was C/5 (C1=150 mA/g). The cycling wasperformed for 32 cycles between 3.0 and 4.4V. As a control group, coincells were also fabricated using untreated LiCoO₂ and tested in the samemanner. The result is disclosed in the graph of FIG. 3.

Referring to FIG. 3, the cells according to the present invention showedan exceptional cycling stability compared to untreated LiCoO₂. Theuntreated LiCoO₂ showed clear degradation, visible by the suppression ofthe voltage profile at the end of discharge, and also showed theincrease of electrical resistance, probably due to the decomposition ofelectrolyte.

Example 10

Impurity-containing LiCo_(0.8)Mn_(0.1)Ni_(0.1)O₂ was prepared by mixinga carbonate precursor, lithium carbonate and lithium sulfate, followedby heat treatment at 1000° C. The carbonate precursor was a mixedcarbonate, obtained by precipitation of transition metal sulfate withsodium carbonate and thus contained a significant level of sodiumimpurity. Chemical analysis and EDS showed thatLiCo_(0.8)Mn_(0.1)Ni_(0.1)O₂ contained about 5% by atoms of sodium and6% by atoms of sulfur per transition metal atom.

The impurity-containing LiCo_(0.8)Mn_(0.1)Ni_(0.1)O₂ as a precursor wascoated with Al—Li—OH—CO₃-hydrate in the same manner as in Example 6. Theexperimental result showed that during the coating process, sulfur andsodium impurities are removed by dissolution into solution so thatLiCo_(0.8)Mn_(0.1)Ni_(0.1)O₂ coated with Al—Li—OH—CO₃-hydrate andpractically free of sodium and sulfur impurities was achieved.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described examples are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the meets and bounds of theclaims, or equivalences of such meets and bounds are therefore intendedto be embraced by the appended claims.

1. A composite precursor for an aluminum-containing lithium transitionmetal oxide, the composite precursor comprising: (a) a lithiumtransition metal oxide core, and (b) an aluminum hydroxide-basedprecipitate layer comprising an active aluminum phase coated on thesurface of the lithium transition metal oxide core, wherein the aluminumhydroxide-based precipitate contains a divalent anion, and wherein thedivalent anion is carbonate and the aluminum hydroxide-based precipitatelayer comprises a lithium-aluminum-carbonate-hydroxide-hydrate.
 2. Thecomposite precursor according to claim 1, wherein the total amount ofaluminum in the precipitate layer is 0.5 to 5% by atoms in respect tothe total amount of transition metal in the complex precursor.
 3. Thecomposite precursor according to claim 1, wherein the lithium transitionmetal oxide as the core of composite precursor has a layered or spinelcrystal structure.
 4. An aluminum-containing lithium transition metaloxide for a cathode active material of lithium rechargeable battery,produced by heating the composite precursor of claim 1 in the range of500˜1050° C.
 5. The aluminum-containing lithium transition metal oxideaccording to claim 4, wherein the temperature of heat treatment is750˜950° C.